In producing a printed wiring board, a photosensitive film for circuit formation is used to form a resist film for plating or etching. The film is formed by applying a photosensitive resin composition on a first film (base film), drying the applied composition so as to form a photosensitive layer, and then laminating a second film (protective film) on the photosensitive layer. Then, the first film and the photosensitive layer as transfer layers have heretofore been laminated onto a substrate to be laminated of a printed wiring board.
To laminate the conventional photosensitive film for circuit formation, the second film is removed first, and then the transfer layer is placed on the substrate such that the photosensitive layer faces the substrate. Thereafter, a heating roller is pressed against the first film so as to crimp the transfer layer, in other words, the first film and the photosensitive layer, on the substrate.
To expose the photosensitive layer to light, a negative mask is placed on the first film, and the photosensitive layer is irradiated with a ray of light for exposure through the negative mask. Then, when the photosensitive layer is developed after removal of the negative mask and subsequent removal of the first film, the same pattern as that of the negative mask is obtained on the photosensitive layer. With the photosensitive layer remaining on the substrate as a resist film, a subsequent plating or etching step is carried out.
As the first film, a film (such as a polyethylene terephthalate (PET) film) having a 5% elongation load per unit width (in a longitudinal direction) at 80° C. of not lighter than 100 g/mm is used. Its thickness is generally about 20 μm. The first film must have such a thickness so as to increase the tensile strength of the photosensitive film for circuit formation. Further, the film must have hardness to some extent.
The photosensitive layer is constituted by a photosensitive material characterized in that when irradiated with ultraviolet radiation or the like, physical properties of irradiated portions change. As the photosensitive material, a suitable composition is selected according to application purposes. The thickness of the photosensitive layer is set at, for example, 25, 33, 40 or 50 μm, according to needs.
The second film is a protective film which may comprise a polyethylene and have a thickness of, for example, 30 μm.
The transfer layer must conform to pits and projections on the substrate when laminated so as to avoid occurrence of non-crimped portions between the photosensitive layer and the substrate.
In recent years, an increase in density of wiring of a printed wiring board is underway, and high resolution is demanded. To achieve high resolution of the photosensitive film for circuit formation, it is effective to decrease the thickness of the photosensitive layer. However, since the amount of the photosensitive layer to conform to pits and projections on the surface of the substrate is decreased, the conventional photosensitive film for circuit formation has many non-crimped portions between the substrate and the transfer layer. Therefore it cause a problem that a satisfactory yield cannot be obtained. Further, since the first film in the conventional photosensitive film for circuit formation must have the foregoing thickness and hardness, the flexibility of the whole transfer layer is insufficient. Therefore, it is difficult for the transfer layer to conform to pits and projections on the surface of the substrate in order to laminate completely. As a result, many non-crimped portions occur between the substrate and the transfer layer, causing a problem that a satisfactory yield cannot be obtained.
To overcome these problems, a variety of methods have heretofore been proposed. For example, there is a method in which the photosensitive film for circuit formation is laminated on the substrate after water is applied to the substrate (See Japanese Patent Application Laid-Open Nos. 1982-21890 and 1982-21891).
In this method, since a thin water layer is applied uniformly, the surface of the substrate must be cleaned. Further, when a small-diameter through hole is present, water collected in the through hole is liable to cause a reaction with the photosensitive layer, causing a problem such as a reduction in developability.
Further, there is also proposed a method in which the photosensitive film for circuit formation is laminated on the substrate after a liquid resin is laminated on the substrate so as to form an adhesive intermediate layer (See Japanese Patent Application Laid-Open No.1977-154363).
This method has problems such as reductions in developability and removability with respect to a small-diameter through hole and an increase in costs caused by application of the liquid resin.
In addition, there is also known a method by which lamination is carried out under a reduced pressure by use of a vacuum laminator (See Japanese Patent Publication No. 1978-31670 and Japanese Patent Application Laid-Open No. 1976-63702).
This method is rarely used in general circuit formation since it requires an expensive device and it takes time to produce a vacuum. The method is used only for lamination of a permanent mask used after formation of a conductor. In the lamination of the permanent mask as well, a further improvement in conformance to the conductor is desired.
As described above, a photosensitive film for circuit formation which has excellent conformance to a conductor and conforms to an increase in density of wiring of a printed wiring board has heretofore been desired.